The present invention refers to a press plate actuating system, particularly for thermoforming machines during the production of plastic products such as cups, plates, trays, disposable containers, etc.
Basically such machines comprise a vertical structure provided with a mold that is located on a lower plate that is moved towards another upper plate, fixed or movable, for molding a continuous thermoplastic film. The lower plate pulls the mold downward to allow the thermoformed product to be removed, also to position said film for molding a new product; the lower plate pushes the mold upward towards the upper plate that contains a knife or a counter-knife, as well as eventually towards an auxiliary counter-mold the function of which is to assure that the walls of the thermoformed product are as uniform as possible, forcing the plastic material film to assume the shape of the mold, with the aid of temperature and pressure and/or vacuum.
Thus, when the thermoplastic film is heated, it melts down and conforms to the molds by applying compressed air or vacuum. After the product is molded, the mold itself cuts the product and separates it from the rest of the film.
Also, this downward movement is necessary so that the product may be removed from the mold. The upward and downward movements should be carried out as fast as possible so as to favor high production. Besides the fast upward and downward movements, the plate should be strong enough to support the molding compressed air, as well as to cut the products by detaching them from the plastic film by the end of the upward movement. According to the materials, such as PET (polyethylene terephthalate), the force applied is quite strong.
Thus, besides molding, the plate should provide the cutting of the product through an additional pressure of the lower mold towards the upper plate.
Amongst the several known ways through which the plate is driven, reference is made to that carried out by cams; by cranks (also called toggles) driven by pneumatic, hydraulic cylinders, or servomotors; connecting rods (or eccentric axles) driven by engines or servomotors; direct hydraulic cylinders; and the like.
The disadvantage of the cams and hydraulic cylinders is the speed. In the case of the cams the opening or spacing of the plate of the higher mold is accomplished by means of gravity, what restricts its acceleration to the gravity, that is, not allowing accelerations higher than that of the gravity. In the case of hydraulic cylinders, the speeds are limited, besides presenting a number of accuracy problems when it is stopped.
The pneumatically or hydraulically propelled cranks present problems concerning speed limitations, therefore its acceleration in the beginning of the upward movement is not sinusoidal and occurs in an abrupt way.
As to the connecting rods or eccentric axles, used separately for this application, despite having a higher speed and preventing abrupt falls that occur in the cam system, its cutting power is limited, requiring highly powerful and consequently more expensive driving elements.
In order to solve the inconveniences of the prior art, the present invention refers to a press plate actuating system provided with two eccentric axles of different sizes, wherein the first larger eccentric axle moves upward and downwardly the lower plate, and the second smaller eccentric axle promotes the movement that generates the required force for cutting the already molded product. Thus, each of the eccentric axles may be moved at a high speed, without compromising the machine, its parts, or the molded product.
According to the meaning herein, the reference to the eccentric axles that carries out the longer course movement of the lower plate includes similar devices, such as a connecting rod-crank assembly.
Another object of the present invention is to provide an eccentric axle of a reduced course to carry out the cutting, in order to provide a higher cutting force in relation to the one performed by the eccentric axles of a longer course that only carries out the vertical movement.
Another object is to lift and lower said lower plate in the quickest possible way, and at the end of the upward position the second eccentric axle applies a practically instantaneous abrupt cutting force that is the greatest possible.
Advantageously in relation to the prior art, this system provides the speed achieved by connecting rod systems, without its low cutting force limitation, since the second eccentric axles drives the plate with a force that is inversely proportional to the ratio between its dimension and the dimension of the vertical movement eccentric axle. Also, this constructive disposition allows a reduction in the torque of the gear motor that drives the eccentric axles at this same ratio, making it possible to use of smaller gear motors.
Another advantage of this system is that it makes it possible to execute the cutting force at any portion of the course of the smaller eccentric axles, by choosing its relative position. In this embodiment, the cutting force is much higher than the one of the machines that uses only a connecting rod to drive the plate, where it is only possible to cut the film at the end of the course of the connecting rod. The embodiment of the invention allows adjusting the height of the cutting knife electronically, thus making the servomotor that drives the smaller eccentric axle stop at a position suitable for the cut. In machines with a single connecting rod, cranks (toggles) or cams it is necessary to regulate mechanically the height of the upper plate to set the height of the cut.
Schematic figures of particular embodiments of the invention are presented hereinbelow, whose dimensions and ratios are not necessarily the real ones since the purpose thereof is to didactically present the several aspects of the invention, whose scope of protection is determined only by the scope of the attached claims.